Whose limb is it anyway?

September 3, 2008

photo by dolbystereobenIt’s a little weird to think of limb ownership as a field of study. Of course my right arm belongs to me; who else would it belong to?

Our brains process a lot of information coming from our extremities. Beyond the usual sensations of touch – heat, cold, pain, tickling, and so on – we have proprioception, the information telling us where our body is in space. It’s how you can touch your nose even when your eyes are closed.

But when your eyes are open, your brain uses that information too.

Hence the strange effect of the rubber hand illusion , where scientists put a rubber hand on a table in front of someone, and stroke both the rubber hand and the subject’s real hand with a paintbrush. The subject ends up thinking she “owns” the rubber hand. To quote a typically intelligent YouTube commenter:

well duh because theyre seeing it, and they already felt the brush on them so your sense mix up ahahah idk.

photo by stagewhisper

photo by stagewhisper

In fact (and this is something the YouTube commenters were eager to see, although it’s not in the video), if you make stabbing motions toward the rubber hand, the subject will freak out. Scientists can detect activity in the “I’m freaked out” section of the brain. (They have that mapped out, I guess). The subject really thinks the rubber hand is hers.

A new study shows an interesting side effect of “owning” a rubber hand: the subject disowns her existing hand. The researchers found that the real hand’s temperature drops during the experiment, and tactile information from that hand is processed more slowly.

photo by raysto

photo by raysto

Rubber hands sit still and look funny, but it turns out there’s a more convincing way of portraying a fake hand: using a mirror.

If you rig up a mirror so that the subject is looking at, say, their real left hand and a reflection of their left hand, they start to believe they are looking at their left and right hands. The standard setup has the person move, clench, and stretch their hands simultaneously. Watch the video and you’ll see how disconcerting it is when, around 1:55, the demonstrator moves his real hand differently from the reflection. I’m not attached to those hands and I still got weirded out!

A surprising result of all this brain confusion is that it even works for people who are missing a limb. If an amputee thinks that their missing limb is clenched, cramped, or painful, a few minutes with a mirror box can convince their brain that the limb looks and acts normally. In a recent study, after 4 weeks of mirror therapy all of the amputees felt a decrease in their everyday phantom pain.

Why do amputees feel phantom pain at all? Some say it’s a problem with the cut ends of nerves; some say it’s all in their head. One theory, supported by the mirror therapy results, says that the brain is just guessing, in the absence of better information, at what sensation the limb should be feeling. The mirror therapy provides more information, untrue though it may be, leading to relief.

In this fascinating story about itch, mirror therapy helps. Here is the patient’s experience:

The first thing he expressed was disappointment. “It isn’t quite like looking at my left hand,” he said. But then suddenly it was.

“Wow!” he said. “Now, this is odd.”

After a moment or two, I noticed that he had stopped moving his left arm. Yet he reported that he still felt as if it were moving. What’s more, the sensations in it had changed dramatically. For the first time in eleven years, he felt his left hand “snap” back to normal size. He felt the burning pain in his arm diminish. And the itch, too, was dulled.

“This is positively bizarre,” he said.

2 Responses to “Whose limb is it anyway?”

  1. snowgoer said

    I have excrutiating phantom pain. My right leg was amputated above the knee 12 years ago after I already endured 68 operations. I had a degloving injury and was hit by a car 2 years after that in the early 1980’s. my foot feels like it’s on fire being attacked by pirahna. I have tried numerous medications. I am working on contacting Dr. Jack Tsao to see the exact program I should follow for the mirror therapy. Phantom pain is very very real. I wish so badly I didn’t have it because the side effects from the medicines are difficult and the pain interferes with prosthetic comfort.

  2. Mike said

    found this great website with more info on mirror box therapy, it also has a link to a site where you can buy a folding mirror box. http://www.mirrorboxtherapy.com

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